Foam products are employed for a wide variety of commercial applications. Low-cost disposable foams are used for thermal insulation such as insulated cups, trays and clam-shell food holders. Longer lived products such as insulation for thermoses, refrigerators, and freezers and insulated water pipe covers are common products. Rigid foams are now utilized for combined sound and thermal insulation on various residential and commercial dwellings for quick installation. Two part liquid foams are available to fill complex geometries and then solidify. Since some closed-cell foams are very light weight, generally being less than 10 lbs/cubic foot, they serve as long-term life preservers and floatation devices not requiring inflation of air. Some rigid foams exhibit high strength, specifically for cases when strength to weight ratio is critical. The light weight and flexibility characteristics of semi-rigid foams make them ideal for packing materials commonplace in the shipping industry.
One of the major advantages of plastic and plastic foams is its life expectancy. Plastics are durable and do not break down in the environment. Whereas metals are oxidized if not painted and loose their structural characteristics, plastics do not. Metals are reasonably heavy compared to plastics and, in many cases where strength to weight ratio is critical, metals are inferior to structural plastic foams. On the other hand, these long-life and durability properties of plastic have created environmental problems. The United States alone disposes of billions of pounds of plastic annually; plastic foam products constitute a large percentage of this volume. The actual life in the disposal environment is estimated to be in centuries.
Curtailment of this growing landfill problem could be achieved by recycling and/or producing biodegradable materials. However plastic recycling simply has not worked. One reason is the extreme difficulty in separating the various commercial plastics whose number seems to grow weekly. The other factor is the reluctance of the public to participate. Biodegradable plastics are still in the development stage, and to date only poly(hydroxybutyrate)/poly(hydroxyvalerate) [PHB/PHV] has been shown to degrade.
Many people have recognized the need for biodegradable products. Perhaps the first of these was the U.S. Department of Agriculture. In 1980, USDA started work on a starch additive for polymer films to initiate biodegradation. It was hypothesized that the additive would alter the structure to permit biodegradation. The starch additive actually improved some physical properties, but according to Krupp & Jewell, Krupp, L. R. & Jewell, W. J. "Biodegradability of Modified Plastic Films in Controlled Biologic Environments" Environ Sci Tech 26 (1992): 193-198, although the starch additive degraded the biodegradability of the plastic was unaltered.
Other approaches have been presented and patented. There are basically two approaches that have received the most attention. These are 1) photodegradable polymers and 2) production of naturally degradable biopolymers. Active research in these areas was initiated by governments, states, and private industry and individuals. Today commercial plastics incorporating one of these features are available. However, Krupp and Jewell found that only the additives were degraded. The polymer remained unchanged although in some cases it fragmented. Thus the authors concluded that only the organic additives were degraded by the digestive process leaving the polymer intact. There was one exception, the polymer poly(hydroxybutyrate)/poly(hydroxyvalerate). This polymer met all the requirements and was substantially biodegraded by the end of the experiment. Their conclusion was that the addition of organic additives make the plastic film dispersible, not biodegradable.
Recently Tokiwa and Iwamoto patented a biodisintegratable thermoplastic resin foam (Tokiwa Y., et al (May 5, 1992) "Biodegradable Thermoplastic Resin Foam and a Process for Producing Same" U.S. Pat. No. 5,110,838). These inventors produced a foam from a mixed resin consisting in part of a biodegradable resin using the melt kneading process, i.e., bubbles are produced in the bulk plastic with a foaming agent while under temperature and pressure. Upon reduction of pressure and temperature, the bubbles expand to form a foam of specific apparent density. This mixture can be extruded or molded to produce the preferred configuration. The foam would certainly be disintegratable; however, the total material is not biodegradable. Thus the environmental problem has not been solved by this invention, only ameliorated.
Perkins N. B. May 26, (1992) "Method of Making Biodegradable Free Fill Foam Packing Material" U.S. Pat. No. 5,116,550 teaches that biodegradable free fill polyurethane foam can be produced by blending liquid starch or sugars with the polyurethane. This approach is similar to other polyurethane foaming processes in that two components are brought together and chemically reacted thus forming a rigid foam. This product is essentially the same as a standard polyurethane foam except that it does contain biodegradable components. Therefore it is disintegratable.
Miller and Miller, "Plastic Foam Aggregate Matrix Made from Recycled Paper and Fiber Products" U.S. Pat. No. 5,106,880, Apr. 21, 1992) disclose an aggregate plastic foam prepared from a cellulose starch and recycled cellulose fiber material for the purpose of fabricating molded foam products for the shipping industry. The approach uses cellulose starch as a binder to entrap bubbles produced by a gas generating agent within the cellulose filler material. Small aggregates made from cellulose fibers, cellulose starch and bubbles are first formed, presumably to permit drying. The patent does not teach the methodology for producing such a foam. These aggregates are then brought together within a mold with similar materials to form a final foam structure which is both biodegradable and biodistintegratable. This patent is not concerned with syntactic foams.
Syntactic foams have been known for some time. Shannon, in U.S. Pat. No. 3,325,341 discloses a syntactic foam formulated of vitreous or argillaceous spheres bonded together by partial melting with the spaces between the spheres being void or preferably filled with a binder. The Shannon syntactic foam is not readily environmentally dispersible.
Dyksterhouse, et al., U.S. Pat. No. 5,120,769 discloses a syntactic foam which comprises microbubbles defined by glass, ceramic or plastic shells and which are formulated by a method which would appear to leave intersticial spaces between the bubbles other than where they contact one another. The microbubbles are bound to one another by appropriate binders. However, nothing is disclosed about choosing binders which are environmentally degradable so as to make the syntactic foam dispersible.
Cattanach, U.S. Pat. No. 4,876,055 discloses a syntactic foam which comprises microspheres having glass shells which are adhered to one another using a polymeric material (polyetheretherketone). Nothing is disclosed about choosing binders which are environmentally degradable so as to make the syntactic foam dispersible.
Alternative materials which would environmentally disperse and which would very preferably biodegrade in a relatively short period of time would be a significant advance in the state of the art. It would also be desirable if the material could be formed into desired structures without the use of environmentally destructive blowing agents. This invention provides, inter alia, just such materials.